Answer: 'Biased towards', 'biased against' and 'biased in favour of' are all correct. As an English teacher, I find that my students sometimes confuse the concept of 'bias' and think that it is always a bad thing. For example, it is OK to be biased towards your child by loving your child more than you love other children. This is human nature. :-)

I've heard "biased toward" and "biased against".

"Biased toward" or "biased against" work just fine. Some people also use "biased in favor of."